The invention relates to a multi-layer polypropylene structure having exceptionally low film to film coefficient of friction over a wide temperature range.
Highly crystalline polypropylene film is an excellent packaging material, but it has a high film coefficient of friction which makes it difficult to utilize in automatic packaging equipment. This material, in film form, also has a tendency to block at the surfaces under pressure. The blocking tendency is manifest during packaging operations where rolls of film are employed, making it difficult to remove the film from the rolls.
Certain innovations have improved on the surface friction characteristics of highly crystalline polypropylene in film form. For example, in U.S. Pat. No. 3,176,021, it is taught to include minor quantities of fatty acid amides into the polypropylene. In order, however, to obtain the benefits taught by this patent, certain limitations must be observed. The film must be formed from a melt extruded at a temperature between about 400.degree.-550.degree. F. In addition, the amide must be present in from 0.005 to about 2.0 of the polypropylene, and it must be present along with from 0.1 to about 4.0 weight percent of polyethylene. Under these conditions and limitations, the resulting polypropylene film will have a static coefficient of friction no higher than 0.6 which is significantly higher than present day requirements. In addition, such a film does not have the high stereoregularity required by present day packaging demands. Further, it has been found that once said film has been subjected to temperature conditions approaching 140.degree. F., the coefficient of friction increases significantly and is nowhere near the present day requirements of 0.25.
In U.S. Pat. No. 3,399,156, it is taught that the inclusion of the combination of silicone oil and finely divided silica into a polypropylene film produces a film which will have a coefficient of friction from 0.45 to 0.6 and the suitable for use in bread-wrapping machines. This is not a low enough coefficient of friction for use in present day, high speed automatic packaging machinery.
It has been found that to a certain extent high crystallinity in polypropylene impedes facile migration of additives to the surface of such as polymer in film form. This problem has been addressed in copending application Ser. No. 400,830, filed July 22, 1982. This application teaches that by the coextrusion of a comparatively thin skin layer of polypropylene of comparatively low stereoregularity with a core layer of polypropylene of comparatively high stereoregularity, where this core layer contains a surface modifying agent, such as, an amide of fatty acid, the amide will bloom to the surface of the comparatively low stereoregularity polymer with comparative ease.
It is an object of the present invention to provide a multi-layer polypropylene film structure having an extremely low coefficient of friction over a wide temperature range.
It is another object to provide a process for preparing this multi-layer structure.